Certain saw chains use a segment that includes on one side a conventional tie strap or side link and on the other side a modified tie strap or side link which is provided rearwardly with an outwardly offset and upwardly extending arm terminating in an inwardly extending overlying portion with part of the arm and the overlying portion having a forwardly disposed sharpened chisel-like edge giving the overall appearance of a tooth, which tooth is adapted to slice out or cut out the kerf as the saw chain is driven in its endless path about the periphery of the saw or cutter bar.
Other saw chain structures utilize the tooth-like cutter portion in a different manner by mounting same upon the centrally located drive-link segment, with spaced succeeding drive link segments presenting right and left hand tooth-like cutter portions.
Depth gauges or jointers appear in some proposals as integral with the same tie strap or side link that presents the chisel-like tooth. An early example of such an arrangement appears in U.S. Pat. No. 2,508,784.
Other saw chains provide depth gauges or jointers as upward and rearward extensions of centrally located drive links or as part of tie straps or side links located immediately ahead of the cutter link segments. Saw chains exhibiting those characteristics are described and illustrated in U.S. Pat. Nos. 3,329,183, 3,910,148 and 3,951,027.
Saw chain of the type under consideration is intended to be guided and supported as is well known in this field on the periphery of the cutter bar by means of a narrow uniform part peripheral groove formation of substantially uniform depth extending along the upper longitudinal edge of the saw bar around the nose or tip and rearwardly along the lower longitudinal edge.
The end of the saw bar opposed from the nose or tip is suitably slotted for anchoring same upon supporting structure carrying the motor and is recessed in a manner such that the driving sprocket can be mounted closely adjacent such latter mentioned end to engage with the drive links of the saw chain as they emerge from the groove formation all in a well-known manner.
The peripheral configuration of the nose or tip of a cutter bar is curvate and of relatively small radius, limited as it is by the transverse dimension of the saw bar itself. In this region the saw bars can be provided with a suitable sprocket derived from the contour of the driving sprocket to engage driving links as the saw chain is driven around the nose or tip and lift the side links out of contact with the nose which is normally subject to excessive wear should the saw chain contact the saw bar surface itself whereby useful saw bar life is extended.
In other cutter bars the nose is provided with hardened shoulders flanking the curvate groove which resists abrasion and impact and serves to extend the useful life of such cutter bar.
When the chain saw is used for boring the nose or tip is thrust into a work piece such as a log or a tree trunk. In other circumstances the nose or tip can be used in pruning of trees or limbing or in the clearing out of brush.
When a chain saw is used in such latter circumstances the operator is exposed to greater danger than when used for normal bucking or cutting.
The danger arising can be demonstrated by the following: in the case where the side link of the saw chain includes the integral depth gauge spaced ahead of the chisel-like tooth as illustrated in U.S. Pat. No. 2,508,784 the depth gauge extends to an elevation slightly below the upper limit of the sharpened tooth edge to establish the thickness of the wood chip to be severed or sliced from the kerf bottom.
When the saw chain operates in the straight line path along the upper or lower reach of the saw bar it is substantially stable but as each segment presenting the tooth-like cutter moves from the straight line path into the upper curvate path defined by the nose or tip the relative positions of the integral depth gauge and the sharpened chisel-edge of the tooth are altered in a manner such that greater penetration of the sharpened edge can occur. This larger bite immediately increases the load and resistance to passage of the saw chain stalling same and resulting in instantaneous swinging of the saw bar upwardly against the grasp of the operator which powerful thrust can inflict serious injury especially upon the casual or untrained user. This reaction is known as "kick-back". "Kick-back" or reverse thrust can also occur with snagging or catching of branches or twigs in the spacing between the cutter links against the forward surfaces of the depth gauge of a cutter link.
The likelihood of "kick-back" occuring while boring or with impingement of branches or brush and consequent snagging has been reduced by providing guard links mounted to swing upwardly as the chain passes around the curvate nose or extend between succeeding cutter links to fill up the spaces. By suitably contouring such guard links branches or twigs are guided upwardly out of the way.
In other proposals the intermediate drive link of the saw chain is provided with a rearward extension intended to swing from a lower elevation to a higher elevation as the saw chain segments move from the straight line path into the curvate path of the nose or tip. Such extension is adapted to bear against the curved bottom alongside the cutter link so that the separation between the following sharpened edge of the tooth is minimized or even eliminated as the chain passes around the nose.
It is also possible for "kick-back" to occur when the chain saw is used for cutting or bucking. In certain structures of saw chains the cutter link segments can heel or rotate rearwardly and upwardly and force the depth gauge into the kerf or expose the forward snagging surfaces of the integral depth gauge momentarily to small branches even though such surfaces may be protected by the interposed guard links.
The inclusion of an extra guard link or the provision of the rearward extension from the centre link not only tends to increase friction or drag but as well chokes or blocks the free flow of wood chips from the kerf. Moreover, such structures are more costly both from the point of view of manufacturing same and repairing same.
Other saw chains omit the integral depth gauge or jointer and provide rearward extensions of the centre link or the side links in the manner illustrated and described in U.S. Pat. Nos. 3,910,148 and 4,074,604.
The practice in professional use of chain saws is to accept in large part the dangers inherent in "kick-back" since the modifications that have been introduced to lessen the danger increase friction or drag and block or divert the steady flow of chips and lower efficiency and hence work output is decreased.